Refers to someone who watches an event without participating, often with a nuance of passive observation.
Many onlookers gathered at the time of the accident.
He watched the argument as a bystander.
傍観者 specifically means a bystander who watches without getting involved, whereas 第三者 more neutrally denotes any uninvolved outside party.
傍観者 specifically means someone who watches without taking action, while 傍 more broadly refers to anyone outside the situation, acting or not.
傍観者 emphasizes standing by and not getting involved (often figuratively, e.g. in a conflict), while 見物人 simply describes someone watching a sight or event.
傍観者 emphasizes passively watching a situation without acting, while 外野 used figuratively implies onlookers who actively voice opinions from the sidelines.
傍観者 is someone who merely watches from the sidelines without taking responsibility, the opposite of a 担い手 who actively bears and sustains a role.
Compound of 傍 (side, nearby) + 観 (view, observe) + 者 (person). The word literally means 'person who observes from the side'.